main.go

package main

import (
	"bytes"
	"crypto/tls"
	"fmt"
	"math/rand"
	"net"
	"net/textproto"
	"os"
	"os/exec"
	"os/signal"
	"strings"
	"syscall"

	"github.com/chrj/smtpd"
	"github.com/google/uuid"
	"github.com/sirupsen/logrus"
)

func connectionChecker(peer smtpd.Peer) error {
	// This can't panic because we only have TCP listeners
	peerIP := peer.Addr.(*net.TCPAddr).IP

	if len(allowedNets) == 0 {
		// Special case: empty string means allow everything
		return nil
	}

	for _, allowedNet := range allowedNets {
		if allowedNet.Contains(peerIP) {
			return nil
		}
	}

	log.WithFields(logrus.Fields{
		"ip": peerIP,
	}).Warn("Connection refused from address outside of allowed_nets")
	return smtpd.Error{Code: 421, Message: "Denied"}
}

func addrAllowed(addr string, allowedAddrs []string) bool {
	if allowedAddrs == nil {
		// If absent, all addresses are allowed
		return true
	}

	addr = strings.ToLower(addr)

	// Extract optional domain part
	domain := ""
	if idx := strings.LastIndex(addr, "@"); idx != -1 {
		domain = strings.ToLower(addr[idx+1:])
	}

	// Test each address from allowedUsers file
	for _, allowedAddr := range allowedAddrs {
		allowedAddr = strings.ToLower(allowedAddr)

		// Three cases for allowedAddr format:
		if idx := strings.Index(allowedAddr, "@"); idx == -1 {
			// 1. local address (no @) -- must match exactly
			if allowedAddr == addr {
				return true
			}
		} else {
			if idx != 0 {
				// 2. email address (user@domain.com) -- must match exactly
				if allowedAddr == addr {
					return true
				}
			} else {
				// 3. domain (@domain.com) -- must match addr domain
				allowedDomain := allowedAddr[idx+1:]
				if allowedDomain == domain {
					return true
				}
			}
		}
	}

	return false
}

func senderChecker(peer smtpd.Peer, addr string) error {
	// check sender address from auth file if user is authenticated
	if localAuthRequired() && peer.Username != "" {
		user, err := AuthFetch(peer.Username)
		if err != nil {
			// Shouldn't happen: authChecker already validated username+password
			log.WithFields(logrus.Fields{
				"peer":     peer.Addr,
				"username": peer.Username,
			}).WithError(err).Warn("could not fetch auth user")
			return smtpd.Error{Code: 451, Message: "Bad sender address"}
		}

		if !addrAllowed(addr, user.allowedAddresses) {
			log.WithFields(logrus.Fields{
				"peer":           peer.Addr,
				"username":       peer.Username,
				"sender_address": addr,
			}).Warn("sender address not allowed for authenticated user")
			return smtpd.Error{Code: 451, Message: "Bad sender address"}
		}
	}

	if allowedSender == nil {
		// Any sender is permitted
		return nil
	}

	if allowedSender.MatchString(addr) {
		// Permitted by regex
		return nil
	}

	log.WithFields(logrus.Fields{
		"sender_address": addr,
		"peer":           peer.Addr,
	}).Warn("sender address not allowed by allowed_sender pattern")
	return smtpd.Error{Code: 451, Message: "Bad sender address"}
}

func recipientChecker(peer smtpd.Peer, addr string) error {
	if allowedRecipients == nil {
		// Any recipient is permitted
		return nil
	}

	if allowedRecipients.MatchString(addr) {
		// Permitted by regex
		return nil
	}

	log.WithFields(logrus.Fields{
		"peer":              peer.Addr,
		"recipient_address": addr,
	}).Warn("recipient address not allowed by allowed_recipients pattern")
	return smtpd.Error{Code: 451, Message: "Bad recipient address"}
}

func authChecker(peer smtpd.Peer, username string, password string) error {
	err := AuthCheckPassword(username, password)
	if err != nil {
		log.WithFields(logrus.Fields{
			"peer":     peer.Addr,
			"username": username,
		}).WithError(err).Warn("auth error")
		return smtpd.Error{Code: 535, Message: "Authentication credentials invalid"}
	}
	return nil
}

func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
	peerIP := ""
	if addr, ok := peer.Addr.(*net.TCPAddr); ok {
		peerIP = addr.IP.String()
	}

	logger := log.WithFields(logrus.Fields{
		"from": env.Sender,
		"to":   env.Recipients,
		"peer": peerIP,
		"uuid": generateUUID(),
	})

	if *remotesStr == "" && *command == "" {
		logger.Warning("no remote_host or command set; discarding mail")
		return nil
	}

	env.AddReceivedLine(peer)

	if *command != "" {
		cmdLogger := logger.WithField("command", *command)

		var stdout bytes.Buffer
		var stderr bytes.Buffer

		environ := os.Environ()
		environ = append(environ, fmt.Sprintf("%s=%s", "SMTPRELAY_FROM", env.Sender))
		environ = append(environ, fmt.Sprintf("%s=%s", "SMTPRELAY_TO", env.Recipients))
		environ = append(environ, fmt.Sprintf("%s=%s", "SMTPRELAY_PEER", peerIP))

		cmd := exec.Cmd{
			Env:  environ,
			Path: *command,
		}

		cmd.Stdin = bytes.NewReader(env.Data)
		cmd.Stdout = &stdout
		cmd.Stderr = &stderr

		err := cmd.Run()
		if err != nil {
			cmdLogger.WithError(err).Error(stderr.String())
			return smtpd.Error{Code: 554, Message: "External command failed"}
		}

		cmdLogger.Info("pipe command successful: " + stdout.String())
	}

	for i := len(remotes) - 1; i > 0; i-- {
		j := rand.Intn(i + 1)                           // Generate a random index from 0 to i (inclusive)
		remotes[i], remotes[j] = remotes[j], remotes[i] // Swap elements at index i and j
	}

	var err error
	for _, remote := range remotes {
		logger = logger.WithField("host", remote.Addr)
		logger.Info("delivering mail from peer using smarthost")

		err = SendMail(
			remote,
			env.Sender,
			env.Recipients,
			env.Data,
		)
		if err == nil {
			logger.Debug("delivery successful")
			break
		} else {
			switch err := err.(type) {
			case *textproto.Error:
				logger.WithFields(logrus.Fields{
					"err_code": err.Code,
					"err_msg":  err.Msg,
				}).Error("delivery failed")
			default:
				logger.WithError(err).
					Error("delivery failed")
			}
		}
	}

	if err != nil {
		var smtpError smtpd.Error

		switch err := err.(type) {
		case *textproto.Error:
			smtpError = smtpd.Error{Code: err.Code, Message: err.Msg}
		default:
			smtpError = smtpd.Error{Code: 554, Message: "Forwarding failed"}
		}

		return smtpError
	}

	return nil
}

func generateUUID() string {
	uniqueID, err := uuid.NewRandom()

	if err != nil {
		log.WithError(err).
			Error("could not generate UUIDv4")

		return ""
	}

	return uniqueID.String()
}

func getTLSConfig() *tls.Config {
	// Ciphersuites as defined in stock Go but without 3DES and RC4
	// https://golang.org/src/crypto/tls/cipher_suites.go
	var tlsCipherSuites = []uint16{
		tls.TLS_AES_128_GCM_SHA256,
		tls.TLS_AES_256_GCM_SHA384,
		tls.TLS_CHACHA20_POLY1305_SHA256,
		tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
		tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
		tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
		tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
		tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
		tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
		tls.TLS_RSA_WITH_AES_128_GCM_SHA256, // does not provide PFS
		tls.TLS_RSA_WITH_AES_256_GCM_SHA384, // does not provide PFS
	}

	if *localCert == "" || *localKey == "" {
		log.WithFields(logrus.Fields{
			"cert_file": *localCert,
			"key_file":  *localKey,
		}).Fatal("TLS certificate/key file not defined in config")
	}

	cert, err := tls.LoadX509KeyPair(*localCert, *localKey)
	if err != nil {
		log.WithField("error", err).
			Fatal("cannot load X509 keypair")
	}

	return &tls.Config{
		PreferServerCipherSuites: true,
		MinVersion:               tls.VersionTLS12,
		CipherSuites:             tlsCipherSuites,
		Certificates:             []tls.Certificate{cert},
	}
}

func main() {
	ConfigLoad()

	log.WithField("version", appVersion).
		Debug("starting smtprelay")

	// Load allowed users file
	if localAuthRequired() {
		err := AuthLoadFile(*allowedUsers)
		if err != nil {
			log.WithField("file", *allowedUsers).
				WithError(err).
				Fatal("cannot load allowed users file")
		}
	}

	var servers []*smtpd.Server

	// Create a server for each desired listen address
	for _, listen := range listenAddrs {
		logger := log.WithField("address", listen.address)

		server := &smtpd.Server{
			Hostname:          *hostName,
			WelcomeMessage:    *welcomeMsg,
			ReadTimeout:       readTimeout,
			WriteTimeout:      writeTimeout,
			DataTimeout:       dataTimeout,
			MaxConnections:    *maxConnections,
			MaxMessageSize:    *maxMessageSize,
			MaxRecipients:     *maxRecipients,
			ConnectionChecker: connectionChecker,
			SenderChecker:     senderChecker,
			RecipientChecker:  recipientChecker,
			Handler:           mailHandler,
		}

		if localAuthRequired() {
			server.Authenticator = authChecker
		}

		var lsnr net.Listener
		var err error

		switch listen.protocol {
		case "":
			logger.Info("listening on address")
			lsnr, err = net.Listen("tcp", listen.address)

		case "starttls":
			server.TLSConfig = getTLSConfig()
			server.ForceTLS = *localForceTLS

			logger.Info("listening on address (STARTTLS)")
			lsnr, err = net.Listen("tcp", listen.address)

		case "tls":
			server.TLSConfig = getTLSConfig()

			logger.Info("listening on address (TLS)")
			lsnr, err = tls.Listen("tcp", listen.address, server.TLSConfig)

		default:
			logger.WithField("protocol", listen.protocol).
				Fatal("unknown protocol in listen address")
		}

		if err != nil {
			logger.WithError(err).Fatal("error starting listener")
		}
		servers = append(servers, server)

		go func() {
			server.Serve(lsnr)
		}()
	}

	handleSignals()

	// First close the listeners
	for _, server := range servers {
		logger := log.WithField("address", server.Address())
		logger.Debug("Shutting down server")
		err := server.Shutdown(false)
		if err != nil {
			logger.WithError(err).
				Warning("Shutdown failed")
		}
	}

	// Then wait for the clients to exit
	for _, server := range servers {
		logger := log.WithField("address", server.Address())
		logger.Debug("Waiting for server")
		err := server.Wait()
		if err != nil {
			logger.WithError(err).
				Warning("Wait failed")
		}
	}

	log.Debug("done")
}

func handleSignals() {
	// Wait for SIGINT, SIGQUIT, or SIGTERM
	sigs := make(chan os.Signal, 1)
	signal.Notify(sigs, syscall.SIGINT, syscall.SIGQUIT, syscall.SIGTERM)
	sig := <-sigs

	log.WithField("signal", sig).
		Info("shutting down in response to received signal")
}